Dynamic CT (computed tomography) and MRI (magnetic resonance imaging) perfusion studies are both established techniques to assess hemodynamic parameters in the brain and other organs. Typically, a contrast agent (also known as an imaging agent, indicator or tracer agent) is injected into the venous circulation and the passage of the contrast agent is imaged at discrete times using either CT or MRI. For example, a typical scan may consist of imaging discrete “slices” of predetermined thickness through a brain every 1-2 seconds for 1-2 minutes. This way of imaging the bolus passage is often referred to as bolus tracking.
The usual result of the imaging process is a four-dimensional data set representing an intensity value for each voxel (discretised volume element) of the scan volume at each discrete time. The intensity value can be used to infer the concentration of contrast agent at each time point. The nature of this intensity to concentration mapping depends on the imaging modality and imaging protocol used.
The concentration estimates thus obtained can be used to infer the pathway of flow through the scan volume. An estimate of relative arrival time (also referred to in the art as delay time) of the bolus (contrast agent) at each voxel can be obtained from the concentration data for the voxel, for example by one of the methods described in: Christensen et al. (2008), “Inferring origin of vascular supply from tracer arrival timing patterns using bolus tracking MRI”, Journal of Magnetic Resonance Imaging 27, 1371-1381, the contents of which are hereby incorporated by reference.
The tracking approach outlined by Christensen et al. can be effective in characterising flow in the periphery of the vasculature, where blood flow velocity is lower and delay differences between adjacent voxels are consequently larger. However, it is impractical in characterising flow in the large cerebral arteries. Blood flow velocity in the large cerebral arteries is on the order of 50 cm/s, translating into arrival time differences of approximately 4 ms between two adjacent voxels which are spaced 2 mm apart (such spacings being typical in the art). These arrival time differences are orders of magnitude lower than the sampling rates of 1-2 s presently used in bolus tracking, and thus, may be difficult or impossible to characterise directly using previously known methods.
It would be desirable to overcome or alleviate the limitations with known methods as described above, or at least to provide a useful alternative.